This invention relates to improved metal halide lamps.
Conventional metal halide lamps contain an electric light source comprising an arc discharge tube made of a vitreous material such as quartz or a high temperature glass, which is generally centrally disposed within a vitreous outer envelope and supported by a metal frame. The outer envelope generally has a stem or neck shaped portion on at least one end thereof, which terminates in a substantially metal base portion. The arc discharge tube or xe2x80x9carc tubexe2x80x9d accommodated in the envelope is connected to the metal base by current supply conductors. The arc tube contains an electrode disposed at each end and contains a fill comprising mercury, a halide of sodium and a halide of one or more metals such as scandium, cesium, calcium, cadmium, zinc, barium, mercury, gallium, indium, thallium, germanium, tin, thorium, selenium, tellurium, etc. Usually, the arc tube also contains an inert gas such as argon. The discharge vessel is usually accommodated in a nitrogen filled tubular or ovoid outer envelope.
Increasing the lumen output for quartz metal halide lamps has been one of the major subjects in lamp research and development. Many improvements in the past have involved arc tube design changes such as arc tube geometry, chemical fillings, electrode dimensions, and wall loading. All of such improvements have used nitrogen or other inert gas filling in the outer bulbs. Because of high thermal conductivity of the filling gas such as nitrogen, part of the energy is lost through conduction.
Canadian Patent application 2,062,889, discloses that it has long been recognized that the chemistry encountered in metal halide lamps is such that ultraviolet radiation from the arc tube strikes metal components with the lamp causing the emission of photoelectrons; that under certain conditions, these photoelectrons collect on the outer surface of the arc tube and create a negative potential that attracts the positive sodium ions and accelerates their diffusion through the wall of the arc tube; that the production of such photoelectrons substantially accelerates the depletion of sodium within the arc tube and thus shortens the useful life of the lamp and that a design wherein silicon nitride coatings are employed on the surface of metal lamp components reduce the emission of photoelectrons from such metal lamp components and when deposited on the metal lamp components and the arc tube""s outer surface retard or reduce the loss of sodium from within the arc tube. DE3134907 discloses a similar proposal wherein the outside of the discharge vessel, the metallic points holding this and supplying it with current, and the insulators, or metal parts, and a quartz tube are coated with silicon nitride to prevent alkali loss in metal halide lamps. However, these proposals use nitrogen filling gas so there is no increase in lumen output for the lamps.
There is a continuing need in the art for lamps in which the energy loss is reduced and wherein the lumen output is increased.
An object of the invention is to provide quartz metal halide lamps in which the energy loss is reduced.
Another object of the invention is to provide quartz metal halide lamps in which the energy loss is reduced and the lumen output is increased.
These and other objects of the invention are accomplished, according to a first embodiment of the invention in which energy loss is reduced significantly through the use of vacuum lamp outer envelops. The lamp lumen output is increased and up to 3,500 hours, the lamp lumen maintenance, color shift, voltage rise, and CRI shift of lamps produced in accordance with the invention are superior to conventional nitrogen fill lamps.
According to another and preferred embodiment the invention, high lumen output quartz metal halide lamps are provided which comprise a vacuum outer fill, and a silicon nitride CVD coating on the outside of the quartz arc tubes (discharge tubes). Instead of nitrogen filled outers, vacuum lamp outers are used for quartz metal halide lamps to reduce energy loss (since heat conduction loss is reduced) and to increase lumen output. Additionally, only the outside of the arc tubes is coated with silicon nitride, without coating the metal components. This reduces or blocks sodium diffusion through the quartz walls. The silicon nitride coating also retards migration of the trace hydrogen from the lamp outer into the arc tube.
A 10% to 15% increase in lumen output is realized by using a vacuum lamp outer instead of the nitrogen fill outer conventionally used for the traditional quartz fill lamps. The heat conduction loss is reduced and the lamp efficiency is increased significantly.
In especially preferred embodiments of the invention, a silicon nitride coating is employed only on the outside of the arc tubes (the metal components are not coasted) to reduced and/or block sodium diffusion through the quartz wall, and additionally, to achieve a target color temperature and further increase lumens for the lamp, salt ratios are reduced as required. We have discovered that the migration of trace hydrogen from the lamp outer into the arc tube can be retarded by applying the silicon nitride the outside or exterior of the arc tube. Because the sodium migration is blocked or reduced, the other changes in chemical fillings such as adjustment of salt ratios, reduction of excess metal for example, scandium, to reduce the reaction between scandium and quartz, and the addition of thorium iodide to improve the lamp performance may also be applied.